The production of ethylene and propylene, herein referred to as “light olefins” or “prime olefins,” is typically conducted at very large scales to achieve efficient economy of operation, on the order of multiple hundreds of thousands and even multiple millions of metric tonnes per year. This has been a subject of great interest in the emerging field of olefin production via oxygenate conversion with molecular sieve catalysts, especially silicoaluminophosphate (SAPO) molecular sieves, and a number methods have been provided to achieve large volume production with the minimum amount of equipment. However, these methods have had to manage certain characteristics associated with catalysis, most notably rather low relative activities required to obtain desirable reaction selectivities. EP1299504A2 and US2004/0104148 are exemplary references in this regard, resorting to multiple reaction zone conduits or irregular geometries.
When converting oxygenates to a light olefin product, it has been problematic to maximize the production of light olefins, and to control, typically to minimize, the production of by-products, such as light saturates and C5+ compounds. In conventional oxygenate conversion processes, high pressure conversion is problematic in terms of a resultant poor yield slate. In these conventional processes, high total pressure is used together with relatively low partial pressure of oxygenate, e.g., they call for one to use a large amount of a substantially inert diluent. U.S. Pat. No. 6,441,261 shows poor activity maintenance at high partial pressures of methanol, and recommends using large amounts of a diluent such as steam to achieve a low partial pressure of oxygenate at a high total pressure of reaction to achieve satisfactory catalytic performance. U.S. Pat. No. 5,126,308 calls for the use of non-steam inert diluent to prevent long term loss of inherent catalyst activity at total reactor pressures up to 250 psig, but provides no catalytic performance data at relatively high oxygenate partial pressures. U.S. Pat. No. 5,811,621 notes total reactor pressures of up to 20 atmospheres, but calls for staged injection of methanol through a series of individual reactors to maintain very low oxygenate partial pressures, an expensive prospect given the very low productivity of olefins for a given volume of reactor and catalyst.
U.S. Pat. No. 4,814,541 discusses conducting high pressure reactions with oxygenates in a slurry of low volatility, high molecular weight diluent, and provide limited performance data for such systems further comprising large amounts of water diluent. While the partial pressure of oxygenate in the data provided is relatively high, the disclosed conversions are quite low. Typically, relative activity declines significantly with increasing oxygenate partial pressure in traditional reaction techniques.
Other methods of conducting oxygenate conversion with silicoaluminophosphate (SAPO) catalysts have been disclosed using high partial pressures of oxygenates, such as U.S. Pat. No. 6,531,639, which indicates advantages for increasing reaction WHSV when increasing oxygenate partial pressure. However, the data provided in that patent also show the relative activity decline of the SAPO catalyst with increasing partial pressure.
Other references such as U.S. Pat. No. 6,673,978, disclose an increase the residence time of catalyst in the reaction zone relative to that in the circulation zone because of detrimental catalyst degradation products generated in the circulation zone. Moreover, U.S. Pat. No. 6,613,950, which similarly directs one increase the residence time of catalyst in the reaction zone relative to that in the circulation zone in order to decrease coke make, also discloses the use of catalysts of very high acidity, that is, having a very high Si/Al2 ratio, which can lead to high coke yields.
It would be desirable to produce as much olefin as practical through a given volume of the reactor while maintaining olefin selectivity, thus higher oxygenate concentrations. The operation of an oxygenate conversion reaction at relatively high oxygenate partial pressures, e.g., in excess of 20 psia, and particularly in excess of 40 psia, would be of great interest if desirable reaction yields could be achieved. It would also be desirable to increase activity maintenance over the course of an oxygenate conversion reaction, e.g. by reducing coke yield. In particular, increasing apparent catalyst activity at a given modest acidity would be desired without resorting to increasing alumina (Si/Al2) ratios, which while increasing inherent catalyst activity tends to provide excessive coke yields. Solving this problem would allow for less catalyst needed and much lower catalyst circulation rates, which would decrease the physical attrition of catalyst and allow for greater production of desired olefins with a lower number of catalyst circulation conduits. Solving both problems simultaneously would provide for a more cost effective and potentially much simpler reaction system with an advantageous yield slate.